What is the cost of 400g at £1.50 per 1/2kg

Well 0.5kg is an awkward number, so let's double £1.50 and 0.5kg to get £3 for 1kg, or 1000g. 

So now, we can set up a proportion. (x is the cost) 

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Now we multiply 400 on both sides to get 'x' on its own.

 × 

So now, we know the cost for 400g = £1.20.